The affidavit describes 16 other letters signed "The Bishop."
Exactly why the letters were signed "The Bishop" was not clear, authorities said.
One theory was that Tomkins was inspired by a Charles Bronson movie, "The Mechanic," in which Bronson played an assassin named Arthur Bishop, who leaves a note reading, "Bang, you're dead." Federal officials, however, indicated they had no evidence to suggest Tomkins was mimicking the movie.
One letter was postmarked Palatine, another Chicago suburb, on June 9, 2006, addressed to investment management executives, and read: "TIMES UP ... IT IS BETTER TO REIGN IN HELL, THAN TO SERVE IN HEAVEN ... THE BISHOP."
Another postmarked March 13, 2006, was addressed to a senior officer of Navarre, complaining about the executive's pay and a decline in the stock price, officials said. They quoted the letter as saying: "Within the next 60 days you are going to find a way to reverse the downward spiral of the stock price or the devil will be paying you a visit." It ended with the words "tic-toc."
Sales records from a home improvement store in Dubuque show that in December 2006, materials similar to those used in the pipe bombs were purchased with Tomkins' credit card, the affidavit said.
Also, ATM records show Tomkins' account was used near some of the places where threatening letters were sent, officials said.
Officials cited records showing Tomkins acquired shares of 3COM Corp. shortly before the first threatening letters were mailed. A letter sent in May 2005 said shares in the Marlborough, Mass.-based company would have to rise $6.66 by the end of October.
In Dubuque, at least four small explosions were heard Wednesday afternoon as federal agents in dark green protective suits searched two storage units at an apartment complex.
Several blocks away, agents also seized items from Tomkins' home, a gray ranch-style house with single-car garage, after bomb experts scoured his neighborhood and blocked off a street.
As postal inspectors removed items from Tomkins' home, a small pony swing blew in a stiff, cool breeze in the back yard. A trampoline was set up nearby.
Neighbor Don Stedman described Tomkins and his children as a pleasant middle American family. He told KDTH-AM he bought Girl Scout cookies from Tomkins' daughters. "They fit in with the neighborhood well as far as we know," Stedman said.
Associated Press writers Heather Hollingsworth in Kansas City, Mo., and Mike Wilson in Dubuque, Iowa, contributed to this report.